The invention concerns a pneumatic infrared radiation detector for intermittent exposure to infrared radiation, consisting of a vacuum chamber having a transparent window of materials from the group calcium fluoride and barium fluoride, and having a metal cup with a highly reflective internal surface forming the back of the chamber, and a flange-like margin sealed to the window with a glass solder, plus a housing surrounding the chamber and a current detector connected to the chamber.
Such detectors can be used for a variety of purposes. For example, they can be used in an infrared gas analyzer for the rapid, selective and continuous analysis of gases and vapors. Furthermore, their use as radiation receivers for radiant heat and for the measurement of reflection in the infrared range has been suggested.
It is generally desired, in this connection, that the permeability of the window to radiation extend as far as possible into the long-wavelength range, for the purpose of increasing the spectral sensitivity of the detector. This requirement is the reason for the use of calcium fluoride or barium fluoride as material for the window. Sodium chloride has still more desirable properties in this regard, but its solubility in water precludes its use in practical applications.
The cup forming the back part of the chamber is to be corrosion resistant and is to have a high reflectivity, because the measuring effect is enhanced by the multiple reflection from the bright surfaces. In known detectors the cup consists of a thick-walled body of base metal to the inside of which a thin layer of gold is applied. The joining together of the cup and the window is accomplished in this case by cementing with a synthetic resin. Such a detector, however, does not fulfill all of its requirements: it cannot be degassed by heating it to temperatures above 400.degree. C. Consequently, a chemically or physically acting absorber for the residual gases must be added to the detector. This, however, entails instability of the gas composition, because the binding of the gas to the absorber is, as a rule, temperature-dependent. The detector must furthermore be usable for a great number of common gases, such as carbon monoxide, carbon dioxide, water vapor, sulfur dioxide, nitric oxide, and a great number of hydrocarbons. However, no absorber has yet become known which would be compatible with all of the gases involved. The synthetic resin adhesive, consisting for example of epoxy resin, is sufficiently permeable to water vapor and carbon dioxide to interfere with the application in question.
On account of the known disadvantage of the cementing of the cup and window, attempts have been made to use a glass solder in making the joint. It is obvious that such a glass solder must be one which has the same coefficient of expansion as the material of the window. The coefficient of expansion of calcium fluoride is 235.multidot.10.sup.-7 /K. The coefficient of expansion of barium fluoride is 185.multidot.10.sup.-7 /K. Glass solders having corresponding coefficients of expansion, however, must necessarily have a high content of alkali metal oxides. They are therefore soluble in water and dissolve relatively rapidly in a humid atmosphere. That a detector may be used in a humid atmosphere is not unlikely. Also, water vapor could not be measured with such a detector.